In the media info panel, there is a group called "Annotations". As I understand, its subdivisions (Label, Rating,...) are predefined international settings. But can I make changes in here? Is there a way to add, rename or delete some of these subdivisions?

1. Specify which fields are displayed by using the Show/Hide Fields tool. (Open the tool by clicking the tiny Configure List icon at the upper right of the Info pane and selecting Show/Hide Fields....)

2. Create Custom Fields if the default fields are not sufficient for your needs. Select Edit > Custom Fields. I believe the maximum number of possible custom fields is 15.

Also, note that the default field names vary depending on the Field Name type you have selected in Preferences: General. You can choose either IPTC core, Microsoft Expression Media, or Adobe Photoshop CS.